Conventional mercury switches generally consist of a pool of liquid mercury trapped within a closed vessel having electrical insulation among the conductive contacts or electrodes. The pool of liquid mercury establishes a conducting path between electrodes for certain switch orientations, depending upon the degree of imbalance within the vessel. Mercury switch electrodes are typically made of metals such as copper, which are easily wetted by liquid mercury and have a low electrical contact resistance. When the distance between the electrodes is small, the choice of copper as an electrode material is unsatisfactory. As a result of wetting, mercury amalgamates with the electrode metal and solid particles of mercury compounds form on the electrodes, thus modifying their switching characteristics, usually making the switch unreliable. In addition, these particles of mercury compounds can break off and enter or float in the liquid mercury pool, causing shorting between the electrodes.
Attempts have been made in the art of making mercury switches to solve the wetting problem, but none has been entirely successful. U.S. Pat. No. 1,971,924, to Walker, discloses the use of a chrome-iron material as its electrode coating. Iron, however, forms an iron-mercury amalgam that causes wetting. Walker, in fact, recognizes the wetting problem but relies on its being small in magnitude. Further, the presence of chromium could provide a very thin oxide which would not survive under abrasion or in a hydrogen environment. Experiments have indicated that stainless steel type 304 forms such an oxide and will be wetted under such circumstances.
U.S. Pat. No. 1,744,109, to Phelan, discloses the use of molybdenum as electrodes, and once again he accepts a small amount of wetting, as is discussed in the specification.
Specifically, when the electrode is made of the material mentioned above, it "does not readily amalgamate with or become wetted by the mercury and, therefore, only a small film of mercury, if any, adheres to the body portion 21" (Emphasis added). Actually, it is not the mercury that adheres; mercury compounds are formed.
In U.S. Pat. No. 2,133,986, Green depends upon two and one-quarter weight percent beryllium to impart non-wetting characteristics to copper. Green asserts that amalgamation is acceptable provided there is no dissolution of the electrode. This concept is rather flawed since dissolution cannot be avoided in the process of amalgamation.
Finally, U.S. Pat. No. 4,311,769, to Andreev et al., describes a situation where the electrodes are purposely wetted with a surface layer of mercury, a situation which would be intolerable for many applications due to the bridging that it ultimately causes.
Thus, in the background art related to mercury switches, two common ideas appear: (1) the problem of wetting is recognized, but is tolerated when small in magnitude, and (2) attempts are made to actually wet the electrodes, thus compromising the requirement of non-wettable electrodes.